Remove stdio's implicit init and require client.connect universally

[atesgoral]

From discussion: #327 (comment)

To minimize user impact while avoiding a breaking change, proceeding in the following steps looks appropriate:

• Support only the addition of the client.connect method Add client-level connect handshake to stdio transport #336
• Show a deprecation warning if client.connect has not been called in advance Warn on implicit stdio initialization #338
• Raise an error if client.connect has not been called in advance

[koic]

It's my understanding that #327 does not implement connect for the stdio client. I've opened #336 as Phase 1.